The present invention relates to a method of detecting spark plug fouling in an internal combustion engine. The present invention further relates to an ignition system having means for carrying out such a method.
In an internal combustion engine, an air-fuel mixture introduced into a cylinder is ignited by a spark produced at a spark gap between a center electrode and a ground electrode of a spark plug provided to the cylinder. As shown in FIGS. 12A and 12B, a spark plug 17 includes a metal shell 17d, an insulator 17c enclosed in the metal shell 17d, a center electrode 17a insulated by the insulator 17d from the metal shell 17d and having an end portion protruding from the insulator 17d, and a ground electrode 17b having an end attached to the metal shell 17d and the other end opposed to the end portion of the center electrode 17a. Such a spark plug 17 is constructed so that the insulation resistance between the center electrode 17a and the ground electrode 17b (i.e., the insulation resistance of the portion schematically represented by a voltmeter V in FIGS. 12A and 12B) is sufficiently large.
In such a spark plug 17, there can occur such a case in which when a rich mixture is introduced into a cylinder, the mixture is not combusted completely due to a factor such as incomplete atomization of fuel, and so-called carbon fouling (i.e., deposition of carbon or black soot on the surface of insulator 17c) is caused. When the amount of carbon adhered to the surface of the insulator 17c becomes large, that is, when the progress of carbon fouling becomes noticeable, the insulation resistance between the electrodes 17a and 17b of the spark plug 17 becomes smaller, thus possibly causing such a case in which when a high voltage for ignition is applied to the spark plug 17 from an ignition coil (not shown) to produce a spark at the spark gap g, leakage current flows through the deposition of carbon C so that a spark is not produced but a misfire is caused.
Thus, it has been proposed such a spark plug fouling detecting method that utilizes a technique of detecting ion in terms of ion current, which ion is generated when an air-fuel mixture is ignited by a spark plug and combusted, as disclosed in Japanese Patent Provisional Publication Nos. 11-13620 and 11-50941. A leakage current due to spark plug fouling is superimposed on an ion current so that the behavior of current detected by an ion current detecting means (ion current detecting circuit) at the time of generation of ion current (more specifically, the behavior of current after the focusing of ion current) varies depending upon a variation of leakage current which is caused to vary depending upon the progress of spark plug fouling. The method disclosed in the above described publications is adapted to detect the progress of spark plug fouling by monitoring the behavior of the current detected by the ion current detecting means.
In the meantime, as shown in FIG. 12A, even when spark plug fouling has been caused though the progress in adherence of carbon (black soot) C to the surface of the insulator 17c is at a stage prior to causing a short circuit between the electrodes 17a and 17b of the spark plug 17, there may occur such a case in which a sufficient insulator resistance is still kept between the electrodes 17a and 17b. In this connection, there may further occur such a case in which when a high voltage for ignition is applied from an ignition coil to the spark plug 17, the high voltage does not jump across the spark gap g to create a spark but the carbon C adhered to the surface of the insulator 17c conducts the current delivered to the spark plug 17 (i.e., carbon serves as a discharge path) to cause the high voltage to jump across a gap between an end portion of the carbon layer C and the inner wall surface of the metal shell 17d to create a spark which is so-called interior jumping or leak spark to inner shell bore. Although the mixture can be ignited if located adjacent a flame kernel produced by the interior jumping, such a spark by interior jumping is more difficult to be exposed to the mixture as compared with a spark at the spark gap g, thus resulting in a tendency that the combustion efficiency attained by the interior jumping is lower as compared with that attained by the spark at the spark gap g.
However, while the prior art method disclosed in the above described publications is adapted to detect the progress of spark plug fouling, it detects the progress on the basis of leakage current. Generally, the flow of leakage current is caused when the spark plug fouling progresses to such an extent as to cause a short circuit(i.e., carbon is adhered to the surface of an insulator to such an extend as to cause a short circuit between the electrodes of the spark plug) and the insulation resistance between the electrodes is lowered. The method of the above described publications can detect such spark plug fouling that has progressed to such an extend as to cause a short circuit between the electrodes of the spark plug, i.e., such spark plug fouling that is considered to be in a condition of causing misfires in a high probability, but cannot detect such spark plug fouling that has not progressed to such an extent as to cause a short circuit between the electrodes of the spark plug (i.e., the progress of spark plug fouling is at a stage prior to causing a short circuit between the electrodes) but to such an extend as to be capable of causing interior jumping.
It is accordingly an object of the present invention to provide a spark plug fouling detecting method which can detect such spark plug fouling that is causative of interior jumping, and therefore can detect such spark plug fouling at a stage of progress prior to a stage of causing a short circuit between the electrodes of the spark plug.
It is a further object of the present invention to provide an ignition system for an internal combustion engine, which has means for carrying out a spark plug fouling detecting method of the foregoing character.
To accomplish the above object, the present invention provides a method of detecting spark plug fouling in an internal combustion engine. The engine has an ignition system that interrupts flow of primary current through a primary winding of an ignition coil and thereby inducing a high voltage for ignition in a secondary winding of ignition coil and applies the high voltage for ignition to a spark plug. The method comprises detecting a discharge current flowing between electrodes of the spark plug when the high voltage for ignition is applied to the spark plug, and determining a fouled condition of the spark plug on the basis of the discharge current.
In a spark plug provided to a cylinder of an internal combustion engine, a discharge current flows between the electrodes of the spark plug when a high voltage for ignition generated by an ignition coil is applied to the spark plug. It is considered that there are two kinds of discharge produced by the spark plug, i.e., discharge that is produced at the normal spark gap (hereinafter referred to as xe2x80x9cnormal dischargexe2x80x9d), and discharge that is produced due to conduction of current by a layer of carbon adhered to the surface of an insulator of the spark plug, namely, that is produced by so-called interior jumping. In this connection, at the time of interior jumping, the discharge current flows through a discharge path constituted by the layer of carbon adhered to the surface of the insulator and having a relatively large resistance. For this reason, the discharge current flowing between the electrodes of the spark plug at the time of interior jumping, differs in a current value from the discharge current flowing between the electrodes of the spark plug at the time of normal discharge. Thus, by monitoring the discharge current at the time of discharge of the spark plug, it becomes possible to make judgment on whether normal discharge or interior jumping is caused by the spark plug.
The interior jumping is caused at a stage of the progress of fouling prior to the stage in which the electrodes of the spark plug are shorted by adherence of carbon. Thus, by detecting the occurrence of interior jumping, it becomes possible to detect spark plug fouling at a stage of progress prior to a stage in which the electrodes of the spark plug are shorted by adherence of carbon.
The present invention further provides an ignition system for an internal combustion engine comprising an ignition coil having a primary winding and a secondary winding, a spark plug having a pair of electrode and an insulator insulating between the electrodes, and a control unit that interrupts flow of primary current through the primary winding and thereby inducing a high voltage for ignition in the secondary winding, wherein the control unit includes means for detecting a discharge current flowing between the electrodes of the spark plug when the high voltage for ignition is applied to the spark plug, and means for judging if said spark plug has been fouled on the basis of the discharge current.